This invention relates to a communication network system, and more particularly to a method of controlling the operation of stations in a distributed network system in which a plurality of stations are linked through a ring (or loop) transmission path.
A typical data communication system for a plurality of stations is known as a token ring network, as described in an article entitled "Token Ring Access Method and Physical Layer Specifications", IEEE Standard 802.5-1985 (ISO/DP8802/5). According to the above article, all stations included in the network are designed to have the same control logics, and the stations which are equal to one another constitute a network that is controlled distributively. Each station monitors, for example, a token which circulates along the ring transmission path, and if a station does not receive the token for a certain time length or longer, it determines the occurrence of an abnormality in the ring transmission path or in the communication function of any other station, issues an abnormality notice frame called a "beacon" and enters the control operation for locating the fault.
A station which receives a beacon issued by another upstream station on the ring transmission path ceases its generation of a beacon if it has been sending a beacon (beacon send mode), and it enters a status in which it repeats the received beacon by sending ti to the downstream station (beacon repeat mode). Through the operation of the above control logics, the station adjacent on the upstream side and stations capable of normal communication enter the beacon repeat mode one after another, and ultimately only one station located on the immediate downstream side of the fault point retains the beacon send mode. The station in the beacon send mode determines itself to be close to the fault point if it does not receive a beacon within a time period following the time it sent ut a beacon transmission.
After the fault point has been located, the station in the beacon send mode starts a reconfiguration operation for making a ring network which bypasses the fault point. The reconfiguration function was proposed in U.S. Pat. No. 4,763,315 and U.S. Pat. application Ser. No. 171,698.
When a closed ring is completed by the reconfiguration operation, the beacon goes around the ring and returns to the originating station. Upon receiving the beacon, the originating station checks the source address (SA) of the received beacon to judge whether it is the beacon sent by itself. If the station determines the received beacon to originate from itself, it confirms that a closed ring transmission path capable of normal communication is restored, removes the beacon from the ring transmission path, terminates the beacon send operation, creates a new token for the resumption of communication, and sends it over the transmission path. However, the above-mentioned beacon transmission and regeneration of token are carried out even in the case of a missing token due to a short-term network failure which can recover without invoking the reconfiguration operation.
The specifications of the above-mentioned IEEE standard, however, recovery of the network communication function satisfactorily in the following special circumstances.
Supposing a case in which the beacon sourcing station has been taken out of the ring as a result of the reconfiguration, for example, and thereafter the fault is restored, the station which is to remove the beacon placed on the ring no longer exists on the ring and therefore the beacon will continue to circulate. Moreover, all stations on the ring are in the beacon repeat mode and therefore no token is created even though the fault has been restored, leaving the ring communication function which is disabled.
Supposing another case in which more than one fault on the ring has been restored virtually at the same time, beacon sending stations adjacent to the respective fault points enter the beacon repeat mode because all of them can receive beacons sent from other stations. In this situation, each station does not check whether the received beacon is one it has sent, causing the beacons to circulate along the ring, and the problem similar to that mentioned above will arise.
These problems can be dealt with, in general, as a problem of a drifting frame (the above-mentioned beacon) for determining any one of the stations to be a master (or representative) station, i.e., a problem of infinite circulation of a master determination frame on a transmission path, in a distributed network.
The master (or representative) station mentioned here is a station which is located close to a fault point and sends a beacon continuously for a certain duration or longer, or a station such as an Active Monitor for creating a token and monitoring the normality of the token described in the above article which represents a plurality of stations equal to one another included in the network for implementing the control operation for maintaining a normal communication. Methods of resolving the contention of a master station include the one which is used in the above-mentioned beacon control, and the one which uses the comparison of address values. The latter method operates, at the occurrence of contention, to let each station repeat or relay only master station determination frames having source addresses larger than its own address for example, thereby allowing a frame including a maximum source address value to circulate on the ring and eventually determining a unique master station, i.e., one having the maximum address station. This method, however, is not free of the foregoing problems in case a master station determination frame including the maximum address drifts, e.g., when its source station gets out of the ring.
In a centralized control network in which a predetermined central control station controls the network, the above-mentioned problem of the drift of a master station determination frame does not arise. Instead, this problem is inherent is a distributed control network in which a specific station which controls the overall network does not exist and a plurality of stations in the network are provided with equal functions, such as that in which a frame sent over the ring is to be removed by the source station.